Viscoelastic\nand Birefringence Relaxation of Individualized\nCellulose Nanofibers in the Dilute and Semidilute Regions

Viscoelastic relaxation mechanisms of individualized\ncellulose\nnanofibers (iCNFs) dispersed in glycerol in the dilute and semidilute\nregions were investigated by linear viscoelastic and dynamic birefringence\nmeasurements. The birefringence relaxation of the iCNFs was described\nby the orientational and curvature modes of an existing viscoelastic\ntheory for ideal semiflexible polymers (Shankar–Pasquali–Morse\ntheory). However, the Shankar–Pasquali–Morse theory\ncould not fully describe the iCNF viscoelastic relaxation at high\nfrequencies. Considering the results for birefringence relaxation,\nthe experimental tension mode of the iCNFs was evaluated to be higher\nthan the theoretical value. These results show that the viscoelastic\nrelaxations of the iCNFs are different from those of ideal semiflexible\npolymers, in contrast to cellulose nanocrystals (CNCs). As the iCNF\nconcentration increased, the orientational mode dramatically slowed,\nwhich was more drastic than other semiflexible polymers, including\nCNCs. This anomalous behavior is likely due to the nonideal nature\nof iCNFs.